Is My Sealed Jar Safe?

"My jars all sealed; so, they must be safe."

"If it isn't safe, wouldn't it look bad—or wouldn't it smell bad—or wouldn't it taste bad?"

"I must be doing things right because no one has ever gotten sick."

"Grandma did it that way and she never killed anyone."

Extension Educators have heard all the above comments and/or some version of those when people find that scientifically recommended methods differ from what they learned from parents, grandparents, friends, or the internet. So, let's look at each statement.

Why is a Sealed Lid Not a Sign that the Contents Inside Are Safe?

Home food preservation methods are based on destroying microorganisms and enzymes. These can be molds, yeasts, and bacteria that are destroyed by heat from cooking and processing. The most dangerous of the organisms are spores produced by bacteria—especially spores of Clostridium botulinum. Spores are tiny, seed-like units that are produced by living organisms and are inactive until conditions become suitable for reproduction. C. botulinum spores become extremely dangerous when they germinate because they produce a deadly toxin.

If It Looks, Smells, Feels, and Tastes Okay, It Must be Safe, Right?

Indeed, our sensory system will alert us to some types of spoilage. See The Spoilers article for more information.

• We can see some molds, but not all. If bubbles are present, it's a good indication that yeasts have caused fermentation. If the product has turned dark or soft, enzymes have been active, or there was too much air present that caused oxidation. Note:  molds may lower the acidity of the product in the jar and allow C. botulinum spores to germinate.
• If something smells bad, bacterial or yeast activity would be suspected. Flat-sour spoilage, identified by contents smelling sour like vinegar and the food being tasteless, is caused by thermophilic bacteria that survive high temperatures and the product not being cooled in a timely manner. Never allow canned goods to sit in a canner overnight and do allow a few inches of space between jars on the counter for air to circulate. Also, never cover jars with a towel or anything else that will hold heat in when they should be cooling.
• Never taste food you suspect is spoiled. If a canned product has an off-taste, it is likely the work of one of the spoilage organisms. For example, yeasts often produce a sour flavor.  Not all spoiled food will taste bad. 
• The most dangerous of spoilage organisms, C. botulinum, does not have a taste, you cannot see it, and it does not have an odor. Pressure canning recommendations for low-acid meats and vegetables are based on destroying the spores produced by this bacteria before they have a chance of germinating and producing the deadly toxin that causes botulism food poisoning. Never taste a low-acid food that has been improperly canned.

Follow research-tested canning recipes like Penn State Extension's Let's Preserve series.

As always, when in doubt, throw it out!

References

Zepp, M. (2023, September 25). "What is Botulism?". Penn State University.

Zepp, M. (2024, April 30). "The Spoilers". Penn State University.